In general, a three-dimensional GIS consists of two-dimensional space and height dimension or of two-dimensional space and time dimension. In this paper, we propose an algorithm to construct a three-dimensional geographic database which consists of two-dimensional space and spatial precision. The algorithm maintains correct topological structure. Using the three-dimensional geographic database, we can obtain a map of arbitrary scale. The algorithm makes it possible to integrate spatial data of various scales into a unitary spatial data.